No. I.] CONTRIBUTION TO INSECT EMBRYOLOGY. 89 



It is not till after the commissures and connectives are 

 formed that the inter-ganglionic regions become clearly marked 

 out. Throughout the early stages, in fact till the embryo 

 reaches the ventral surface of the q^^ (Stage J), the ganglia 

 are as long as their respective segments and are separated 

 from one another only by the intersegmental constrictions. 

 These have grown very deep in Stage G, especially in the 

 thoracic and abdominal regions. In the median line, as shown 

 in sagittal section (Fig. 29, i.g'^, ^•<^~ )> they form deep tubular 

 ingrowths which may be called furcal pits. Since these pits 

 are median in position they are to be regarded as differentiated 

 interganglionic portions of the neural furrow. They therefore 

 belong to the median cord. They are not found between the 

 mandibular and first maxillary, nor between this and the second 

 maxillary ganglion, and are also wanting between the eighth 

 and ninth, and ninth and tenth abdominal ganglia. Evidently 

 their absence in these cases is due to early fusion to form the 

 infraoesophageal and last abdominal ganglion. In the thoracic 

 segments the furcal pits are converted into chitinous apode- 

 matous structures which give attachment to some of the leg- 

 muscles. It is interesting to note that in the abdomen also 

 furcal pits are distinctly developed as late as Stage K. Here, 

 too, they serve for the attachment of a few weak muscle-like 

 structures, which run from their tips to points in the adjacent 

 abdominal wall, perhaps corresponding to the insertions of the 

 rudimental appendages. Later both muscle-like cords and ab- 

 dominal furcae disappear, — the latter by a very simple process. 

 It will be remembered that at this time the embryo is growing 

 in length and continually covering more and more of the yolk. 

 The tail end is practically fixed at the lower pole of the o^^g, 

 while the head slowly moves upwards. The body-wall is thus 

 stretched in both a longitudinal and lateral direction. Hence 

 the intersegmental constrictions, so deep in Stage J, must 

 gradually become shallower, and the furcal pits, which are 

 nothing but portions of these constrictions, are drawn out from 

 between the connectives to form part of the sternal integu- 

 ment. The stretching not only draws out the folds in the em- 

 bryonic body-wall, but also reduces it to a much thinner layer 



